This invention relates generally to focused ion beam (FIB) etching processes, and more particularly to focused ion beam gas assisted etching (GAE) of silicon and dielectrics based on silicon in integrated circuits (IC).
Focused ion beam processes are widely used for debugging and verification of the functionality of integrated circuits (IC) and for IC circuit editing (CE) applications. Circuit editing involves the modification of individual IC circuits in order to correct design or manufacturing errors that cause IC malfunctions. FIB systems use a finely focused beam of gallium ions that can be operated in a wide range of beam currents from portions of pico-Amperes (pA) to tens of nano-Amperes (nA). Normally, the ion beam is scanned over an area of interest of a microchip producing emission of neutral or charged secondary particles. Neutral particles are mostly atoms and molecules sputtered from the scanned surfaces, and charged secondary particles are mostly secondary electrons and ions. Charged secondary particles are normally used in FIB instruments to form an image of the scanned area, so that its modification can be observed. The main purpose of FIB application for circuit editing (CE) is modification of the scanned area by either selective removal of specific materials (selective etching) or deposition of some specific materials, as, for example, having very low conductivity (insulators) or high conductivity (conductors). FIB systems provide for local “flooding” of a specimen with a variety of different gases. These gases can either interact with the primary gallium beam to provide selective gas assisted accelerated chemical etching or deposition of either conductive or insulating material by decomposition of the precursor gas by the primary ion beam.
As every IC structure comprises multiple layers of metal wiring interconnects and isolating materials (dielectrics), as well as bulk substrate semiconductor material, typically silicon (Si), CE modification typically involves some or all of four basic FIB processes. These are the selective etching of one or more dielectric layers to expose a metal line or circuit of interest, the selective etching of metal to cut an interconnecting line of interest, the deposition of a metal conductor material to connect lines of interest, and the deposition of an insulator material to isolate lines or circuits. Since CE typically involves milling trenches or vias through layers of densely interleaved layers of metals and dielectrics to reach an area of interest, to avoid damage or destruction of the IC it is important that these FIB processes, particularly the etching of dielectrics and metals, be carefully controlled to selectively etch only the target material of interest and to minimize damage to other materials and structures of the IC. It is also important to avoid re-deposition of sputtered material removed by etching onto adjacent trench walls and surfaces of the IC.
An ion beam itself is a destructive agent causing only erosion or sputtering of the exposed solid material of the IC. It is possible to obtain desirable results such as selective etching and deposition of different materials by directing to the target surface where an FIB operation is being performed gaseous precursors for conductor or insulator deposition, or gaseous chemicals for selective gas assisted etching (GAE) of the target material. These chemicals are adsorbed on the surface, and the ion beam activates surface reactions resulting in either deposition or etching of the materials in the area exposed to the ion beam. To obtain a desired pressure of a gaseous agent in the spot of FIB operation, FIB systems use a small nozzle positioned very close to the target surface exposed to the ion beam. The chemical agents are directed to the target spot through this nozzle. In general, the result of any FIB operation depends on the chemistry used and the ratio between chemical pressure and ion dose delivered to the target spot of the operation. FIB systems enable adjustment of local pressure of the chemical agent and ion beam current so that the FIB process produces desirable results with good efficiently.
Currently, xenon difluoride, XeF2, is a widely used FIB dielectric etching agent for silicon dielectrics such as silicon dioxide, SiO2, and silicon nitride, Si3N4. Almost all dielectrics used in IC manufacturing have silicon as one of their base elements. During FIB gas assisted dielectric etching using XeF2, silicon is oxidized by fluorine to form silicon fluoride, SiF4, which is a gas under normal conditions. This affords good efficiency and high selectivity when XeF2 is used for etching dielectrics. Also, the gaseous by-products of the etching process are volatile and easily removed by the vacuum pumping system so that the etched material by-products are not re-deposited on the walls of etched holes and other neighboring surfaces. Since in most cases, etching of a dielectric is used to expose a metal copper or aluminum line, an important advantage of using the fluorinating chemical XeF2 for dielectric etching is that fluorine does not damage copper or aluminum. In contrast to other heavier halogens such as chlorine, bromine and iodine, fluorine does not corrode aluminum and copper deeper than few surface monolayers, which allows opening these materials with minimal damage. The heavier halogens create aluminum compounds that have spongy structure and can sublime with high vapor pressure at room temperature. Thus, traditionally they were used as very fast etchants for aluminum. However, these halogens (except fluorine) corrode copper material deeply which makes them inapplicable for exposing copper. Therefore, the only reasonable option for etching dielectrics in ICs to expose metal lines has been to use a fluorinating agent (rather than halogenating agent) as an etchant, and XeF2 has been used as a dielectric etchant in FIB applications for years.
However, new developments in ICs technology have made XeF2 inapplicable as an etchant in a number of applications. Because of the increasing complexity and density of active elements in ICs, a significant portion of modern ICs employ so called “flip-chip” packaging where access to the metal layers for CE must be obtained only through the transistor layer (or “active silicon”) of the microchip. This means it is necessary to expose the active silicon to FIB chemistries during CE work. Additionally, new “organic” or low-k dielectrics are increasingly being used instead of traditional SiO2, and there is an increasing tendency to employ a variety of other new dielectrics. XeF2 is undesirable and problematic for flip-chip technology and these new materials.
It is very well known that XeF2 corrodes bare silicon, i.e., it spontaneously etches silicon without ion beam assistance. This creates significant difficulties in “back-side” CE operations on flip-chip packaged ICs where active (bare) silicon is exposed, which is inevitable when doing “back-side” circuit edit operations. FIG. 1 illustrates the corrosion and damage of active silicon adjacent to the open metallization in a flip chip packaged IC during FIB etching with XeF2 to expose the metal lines. The vulnerability of bare silicon to XeF2 corrosion requires the application of protective coatings to the silicon prior to XeF2 application. This makes CE operations longer and more complex.
In the case of dense active circuit elements in an IC, where FIB operation has to be performed very close to active silicon, the protective coatings can protect silicon from the top only, but not against natural lateral over-etch. This is illustrated in FIG. 2, which is a diagrammatic transverse view illustrating FIB etching of access holes through a silicon dioxide (SiO2) shallow trench isolation (STI) layer of an IC to gain access to metal lines 20, 21. As shown, to gain access to line 20, the FIB access hole 28 must penetrate an FIB deposited protective dielectric 32 and pass through the SiO2 layer 30 closely adjacent to silicon 34. This exposes the adjacent silicon area to lateral over-etch, by XeF2 as shown at 38. The protective dielectric 32 deposited over the silicon areas may protect the silicon against XeF2 corrosion from the top, but it does not protect it from lateral over-etching at the vertical (in the figure) sidewalls of the access hole.
Furthermore, it has been reported that some types of the new organic dielectrics show signs of corrosion under exposure to XeF2 similar to those observed on silicon. This means that use of XeF2 as a dielectric etchant is problematic not only for back-side CE applications, but also for front-side applications.
It is desirable to afford FIB process etch-assisting chemical compounds that address the foregoing and other problems of known dielectric etchants, that are non-corrosive to metals used for interconnects in ICs and have other desirable properties, and which will react with a solid specimen to produce volatile by-products that can be removed easily from the FIB vacuum chamber so that they are not re-deposited upon the specimen or neighboring surfaces. It is to these ends that the present invention is directed.